Cutaneous leishmaniasis, a vector-borne infectious disease caused by protozoan parasites of the genus Leishmania, is one of the most important neglected infectious diseases worldwide. Currently, 10 million people in 82 (mostly developing) countries are infected. Those infected develop one or more ulcerative skin lesions on the exposed parts of the body, causing serious disability and permanent scarring. Infection of mice with Leishmania major, one of the two main species that causes human cutaneous leishmaniasis, is a well- established experimental model used to study various aspects of immunity in vivo. Most mouse genotypes infected with L. major develop localized lesions that heal spontaneously after a few weeks. However, certain mouse strains - such as BALB/c mice - fail to control infection and develop progressive lesions and systemic disease. The genetic predisposition for susceptibility or resistance to L. major infection in mice strongly correlates with the dominance of an interleukin (IL)-4-driven Th2 response that causes disease versus an IL- 12-driven, interferon-3 (INF-3)-dominated Th1 response that promotes healing and parasite clearance. Mechanistically, INF-3 triggers inducible nitric oxide synthase (iNOS)-operated leishmanicidal systems in macrophages, resulting in the resistant phenotype, while IL-4 inactivates that mechanism resulting in a susceptible phenotype. Recent studies in mice have now defined a critical but previously unrecognized role for IL-4-producing basophils in promoting Th2-type immune responses. Furthermore, a number of studies have reported that IL-3 can promote basophil expansion and enhance basophil cytokine production, and that IL-3 production correlates with susceptibility of BALB/c mice to infection with L. major. However, the precise role of IL-3 and IL-4-producing basophils in cutaneous leishmaniasis has yet to be addressed. We therefore propose to take advantage of exciting opportunities to analyze the role of IL-3 and basophils in a mouse model of cutaneous leishmaniasis. Specifically, we will use BALB/c IL-3-deficient mice, wild-type mice depleted of basophils, and purified populations of bone marrow-derived basophils to test two major but related hypotheses: 1) that IL-3 production exacerbates the severity of disease as reflected by measures of lesion size, parasite burden, T cell cytokine production, and inflammatory effector cell infiltration and function, and 2) that basophils undergo migration to draining lymph nodes and cutaneous lesions in response to infection, become innately activated by L. major antigens, and release Th2-promoting cytokines such as IL-4 that result in BALB/c mice mounting an ineffective Th2 immune response responsible for their susceptibility to infection. PUBLIC HEALTH RELEVANCE: Human cutaneous leishmaniasis continues to rank among the most important infectious diseases on a worldwide basis. The overall goal of this project is to determine the role of white blood cells called basophils and the protein interleukin-3 in the immune response to the protozoan parasite Leishmania major. We believe these studies to be critical to identifying meaningful targets for vaccines, therapeutics, and diagnostics.